Material sheet and process for its preparation

ABSTRACT

The invention relates to a material sheet comprising a woven fabric of polymer tapes, wherein the width of a tape varies less than 2% on average in the longitudinal direction of the tape. The invention also relates to a process for the preparation of the material sheet, and to a ballistic resistant article comprising the material sheet. A ballistic resistant article comprising the material sheet exhibits excellent antiballistic properties.

This application is a divisional of commonly owned copending U.S.application Ser. No. 12/740,475, filed Apr. 29, 2010, which is thenational phase application under 35 USC §371 of PCT/EP2008/009122, filedOct. 29, 2008, which designated the U.S. and claims priority to EPPatent Application No. 07021267.5, filed Oct. 31, 2007, U.S. ProvisionalApplication No. 61/001,096, filed Oct. 31, 2007 and EP PatentApplication No. 08001815.3, filed Jan. 31, 2008, the entire contents ofeach of which are hereby incorporated by reference.

The invention relates to polymeric tapes and a process for thepreparation thereof and further to a material sheet comprising thepolymeric tapes, and to its process of preparation. The invention alsorelates to articles comprising the material sheet, in particular to aballistic resistant article. The invention also relates to differentuses of the polymeric tapes.

A material sheet comprising a consolidated stack of monolayers of aunidirectionally drawn polymer is known from EP 1627719 A1. Thispublication discloses a multilayered material sheet comprising aplurality of unidirectional monolayers consisting of ultra highmolecular weight polyethylene and essentially devoid of bondingmatrices, whereby the draw direction of two subsequent monolayers in thestack differs. The monolayers of the multilayered material disclosed inEP 1627719 A1 are produced by positioning a plurality of tapes of ultrahigh molecular weight polyethylene adjacent to each other wherebyadjacently positioned tapes overlap at least partly along their sideedges. Without the overlap the known multilayered material cannot beproduced.

Although the multilayered material sheet according to EP 1627719 A1shows a satisfactory ballistic performance, this performance can beimproved further.

The object of the present invention is to provide a material sheet thatcan be easily produced and having at least similar properties, inparticular similar antiballistic properties, as the material known fromEP 1627719 A1 or other commercially available materials based onunidirectional PE fibers.

This object is achieved according to the invention by providing amaterial sheet comprising a woven fabric of polymeric tapes, wherein thewidth of a tape varies less than 2% on average in the longitudinaldirection of the tape.

Preferably the polymeric tapes, or simply referred to as tapes, aretapes of a drawn polymer; more preferably, the tapes are unidirectionaltapes of a drawn polymer. With unidirectional tapes is meant in thecontext of the invention tapes which show a preferred orientation of thepolymer chains in one direction, i.e. in the direction of drawing. Suchtapes of a drawn polymer may be produced by drawing said tapes,preferably by uniaxial drawing if unidirectional tapes are to beproduced and which will exhibit anisotropic mechanical properties.

Weaving of tapes and in particular of unidirectional tapes of drawnpolymer is known per se, for instance from WO2006/075961, the content ofwhich is incorporated herein by reference. WO2006/075961 describes amethod for producing a woven material from tape-like warps and weftscomprising the steps of feeding tape-like warps to aid shed formationand fabric take-up; inserting tape-like weft in the shed formed by saidwarps; depositing the inserted tape-like weft at the fabric-fell; andtaking-up the produced woven material; wherein said step of insertingthe tape-like weft involves gripping a weft tape in an essentially flatcondition by means of clamping, and pulling it through the shed. Theinserted weft tape is preferably cut off from its supply source at apredetermined position before being deposited at the fabric-fellposition. While weft tensioning is a necessary condition for processingyarns, it is not desirable when processing tapes. The weaving method andapparatus, as disclosed in WO2006/075961 therefore allows to feed andprocess tape-like warps in a state of low tension. This is achieved bycarrying out weaving in a vertical format because this way the saggingof warps and wefts due to gravity is significantly reduced.

Weaving is conventionally carried out on yarns, having a circle-likecross-section. The conventional weaving elements which directly interactwith the yarns, such as heald-wires, reed and weft transporting meansoften cannot be employed when weaving tapes, since such conventionalelements are designed to handle yarns. Their use in handling tapes willlead to deformation and weakening of the tapes. When weaving tapes andin particular unidirectional tapes therefore, specially designed weavingelements are used. Particularly suitable weaving elements are describedin U.S. Pat. No. 6,450,208, the content of which is also incorporated inthe present application by reference.

The invention further relates to a polymeric tape having a width thatvaries less than 2% on average in the longitudinal direction of thetape. Preferably, said tape is a tape of a drawn polymer, morepreferably, said tape is a unidirectional tape of a drawn polymer.

It was observed that by carefully controlling the width variation of thepolymeric tapes of the invention, a material sheet is obtained with atleast similar properties, in particular antiballistic properties, as theknown material, or other commercially available materials based onunidirectionally aligned PE fibers.

In addition, the material sheet according to the invention is readilyproduced. The conventional material sheet, as described in EP 1627719 A1for instance, is produced by first making a monolayer of a plurality oftapes positioned adjacent to each other, and then applying anothersimilar monolayer at an angle on top of the first monolayer. To give thematerial handling characteristics, adjacently positioned tapes overlapat least partly along their side edges. This process is time consumingand involves more steps than the process to make the material structureof the present invention. With the tapes of the invention, anoverlapping of the tapes is not necessary for obtaining a material sheetwith at least similar properties or handling characteristic, reducingtherefore the number of processing steps. In particular thetape-overlapping step can be dispensed with.

The material sheet of the invention is preferably produced by weaving aplurality of the unidirectional tapes of the invention with theirlongitudinal edges as close as possible to each other, and preferably intouching proximity. This is made possible by using unidirectional tapeshaving a width that varies less than 2% on average in the longitudinaldirection of the tape, as is required by the invention. However, inorder to be able to produce the material sheet of the invention on anindustrial scale at economical speeds, it would be desirable to allow agap between adjacent tapes (i.e. the adjacent tapes in the materialsheets are not in contact along their longitudinal edges—i.e. a gap ofgreater than 0%). Preferably, the material sheet according to theinvention is characterized in that the woven sheet comprises a pluralityof unidirectional tapes of the drawn polymer in the warp and weftdirection, and in that the gap between adjacent tapes in the weft and/orwarp direction is smaller than 10% of the width of the adjacentunidirectional tapes, more preferably smaller than 5% of the width ofthe adjacent unidirectional tapes, even more preferably smaller than 3%of the width of the adjacent unidirectional tapes. Most preferably saidgap is smaller than 1% provided that the inventive tapes have a widthvariation also smaller than 1% on average in the longitudinal directionof the tape.

In a preferred embodiment the tapes of the invention have a widthvarying less than 1% on average in the longitudinal direction of thetape. In an even further preferred embodiment the tapes have a width ofat least 10 mm, more preferably at least 20 mm, most preferably at least40 mm and further having a variation in width which is less than 1% onaverage in the longitudinal direction of the tape. It was observed thata material sheet based on the tapes of this embodiment yields an evenbetter antiballistic performance.

A particularly preferred embodiment of the tape according to theinvention is characterized in that the polymer from which it is made isselected from the group consisting of polyolefins, polyesters, polyvinylalcohols, polyacrylonitriles, polyamides, especially poly(p-phenyleneteraphthalamide), liquid crystalline polymers and ladder-like polymers,such as polybenzimidazole or polybenzoxazole, especiallypoly(1,4-phenylene-2,6-benzobisoxazole), orpoly(2,6-diimidazo[4,5-b-4′,5′-e]pyridinylene-1,4-(2,5-dihydroxy)phenylene).Unidirectional tapes from these polymers are preferably highly oriented,i.e. having a crystallinity as measured by DSC of above 90%, by drawingthe formed material, for instance films, at a suitable temperature.

An even more preferred embodiment of the tape according to the inventionis characterized in that the polymer from which it is made is selectedfrom the group consisting of polyolefins, polyesters, polyvinylalcohols, polyacrylonitriles, and polyamides. The material sheetscomprising these tapes can be very well consolidated.

The material sheet of the invention allows the use of tapes of drawnpolymers, or simply drawn tapes, with relatively low strength, andtherefore does not expressly need high strength drawn tapes made of e.g.ultra high molecular weight polyethylene to obtain good antiballisticperformance. However, in a preferred embodiment thereof the tapes of theinvention comprise ultra high molecular weight polyethylene. The ultrahigh molecular weight polyethylene may be linear or branched, althoughpreferably linear polyethylene is used. Linear polyethylene is hereinunderstood to mean polyethylene with less than 1 side chain per 100carbon atoms, and preferably with less than 1 side chain per 300 carbonatoms; a side chain or branch generally containing at least 10 carbonatoms. Side chains may suitably be measured by FTIR on a 2 mm thickcompression moulded film, as mentioned in e.g. EP 0269151. The linearpolyethylene may further contain up to 5 mol % of one or more otheralkenes that are copolymerisable therewith, such as propene, butene,pentene, 4-methylpentene, octene. Preferably, the linear polyethylene isof high molar mass with an intrinsic viscosity (IV, as determined onsolutions in decalin at 135° C.) of at least 4 dl/g; more preferably ofat least 8 dl/g, most preferably of at least 10 dl/g. Such polyethyleneis also referred to as ultra high molecular weight polyethylene.Intrinsic viscosity is a measure for molecular weight that can moreeasily be determined than actual molar mass parameters like Mn and Mw. Apolyethylene film of this type yields particularly good antiballisticproperties.

The tapes according to the invention may be prepared in the form offilms which is subsequently slit into tapes.

A preferred first process for the preparation of the tape of theinvention comprises forming a polymeric powder bed, compression-mouldingthe polymeric powder bed at a temperature below the melting point of thepolymeric powder, and preferably drawing the compression-mouldedpolymer, and wherein the powder bed is moulded by compression togetherwith at least one compressible bordering means positioned onto thepowder bed. The invention also relates to such a process.

Preferably, the powder bed is compression-moulded together with at leasttwo parallel compressible bordering means, said means defining in thepowder bed an in-boundary part and an out-of-boundary part or parts. Themelting point of the polymeric powder, also called melting temperatureis determined by DSC as detailed below.

By compressible bordering means it is understood means made of acompressible material, said means partitioning the powder bed into atleast two parts. By compressible is meant that the means do notsubstantially interfere with the compression moulding of the polymericpowder bed. In a preferred embodiment, the compressible bordering meansare compressible strips. In particular, the undeformed strips typicallyonly take a negligible part of the compressive pressure produced by thecompression means of the compression-moulding device, preferably lessthan 5%, more preferably less than 2%, most preferably less than 1%. Thestrips act as a firm boundary for the polymeric powder and it has turnedout that the use of said strips provides for a polymeric powder bed thatis well controlled and of substantially uniform distribution, at leastbetween the boundaries formed by the at least two strips.

The compressible bordering means and in particular the strips, may bemanufactured from any material that is flexible enough to provide forthe desired compressibility. Preferred materials include thermoplasticpolymers, of which polyolefins, such as polypropylene are particularlypreferred. Another particularly preferred material comprises a rubberpolymer, and more preferably a high temperature resistant rubberpolymer, such as a silicon rubber. The means, e.g. strips, arepreferably made of a material having a melting temperature as determinedby DSC of at least 10° C., more preferably at least 20° C., and mostpreferably at least 30° C. higher than the melting temperature of thepolymeric powder.

Said means, and in particular the strips, may have any shape. It ispossible for the means, e.g. strips, to for instance have a rectangular,triangular, circular, or polygonal cross-section, whereby the means orstrips can be solid or hollow. Preferably, strips are used that arehollow since such strips are easily compressible. In a particularlypreferred process according to the invention, the strips comprise ahose, or a tube. The undeformed height of the means, e.g. strips, can bevaried within large boundaries. The undeformed height of the means, e.g.strips, is preferably equal or higher than the undeformed averagethickness of the polymeric powder bed. In the event of a hollow stripsuch as e.g. hose or a tube, the ratio of outer diameter: inner diameterpreferably is 3:2, more preferably 3:1.5 and even more preferably 3:1.

It is also possible, according to the invention to provide strips withinthe polymeric powder bed having a lower undeformed height than theundeformed average thickness of the polymeric powder bed. Such stripsresult in embossed tapes having locally decreased thickness.

In a preferred embodiment of the process of making the tapes of theinvention, the compression-moulding process is carried out according tothe process of U.S. Pat. No. 5,091,133, the disclosure thereof beingincluded herein by reference. Therefore, the invention relates to aprocess for the preparation of a polymeric tape, the process comprisingfeeding a polymeric powder between a combination of endless belts thusforming a polymeric powder bed, compression-moulding the polymericpowder bed at a temperature below the melting point of the polymericpowder, conveying the resultant compression-moulded polymer between theendless belts and preferably drawing the compression-moulded polymer,wherein at least two strips of compressible material are fed andconveyed between the endless belts together with the polymeric powder.According to this preferred embodiment, it is essential that the stripsare compressible, at least in their undeformed state, by which is meantthat the strips do not substantially interfere with the compressionmoulding of the polymeric powder bed.

Preferably, the strips are arranged substantially parallel to each otherand in the conveying direction, whereby the strips are arranged in sucha way that the width of the resulting tape varies less than 2% onaverage in the longitudinal direction of the tape. This may suitably bedone by e.g. a creating a fixed distance between the strips by feedingthe strips through a rack with a predefined width for the strips.Preferably two of such racks are present for best alignment of thestrips, e.g. such racks are present before and after a conveyingsection. More preferably the strips are fed through the said rack whileunder tension for alignment. The required tension can be easily bedetermined by routine experimentation, whereby too high a tension couldlead to excessive deformation of the strips and too low a tension willnot result in a tape with less than 2% variation of width. Analternative method could be electronically controlled width-gaugesbetween and guiding the strips. According to this embodiment, thepolymeric powder is fed or scattered onto the belt over some width,whereby the width is generally larger than the distance between thestrips. The powder bed therefore overlaps with the strips. In otherwords, the strips are arranged such that they extend within thepolymeric powder bed along the outer edges thereof, and at some distancefrom the outer edges. The polymeric powder bed is in this way divided ina part that is in-boundary and extends between the strips, and in a partthat is out-of-boundary, the latter part extending from the strips tothe outer edges of the powder bed. Preferably, the compression-mouldedout-of-boundary part(s) of the powder bed may be removed and recycled.Such process therefore offers the possibility to produce a polymerictape substantially without waste.

A further particularly preferred embodiment of the process of making thetapes of the invention is characterized in that the number of strips is2, and are used to create an in-boundary part between these 2 strips andan out-of boundary part or parts, whereby the strips are arranged suchthat they longitudinally extend within the polymeric powder bed alongthe outer edges of said bed such that the width of the out-of boundarypart(s) of the powder bed does not exceed 30% of the total width of thepowder bed. When scattering polymeric powder onto e.g. a belt, the sideregions thereof will generally show a variation in thickness, thethickness decreasing towards the sides of the powder bed. It has turnedout that by positioning the strips according to this embodiment, i.e.such that the width of the out-of boundary part(s) of the powder beddoes not exceed 30% of the total width of the powder bed, thein-boundary part of the powder bed will have a substantially uniformthickness. With the “width of the out-of-boundary parts” is meant thetotal width of the out-of-boundary part or parts. It is believed thatthe more uniform thickness of the in-boundary part of the powder bed isresponsible for the observed improved properties of the final polymerictape.

In an even more preferred embodiment of the process of making the tapesof the invention, the strips are positioned at a distance from the outeredges of the polymeric powder bed of at most 20% of the total width ofthe polymeric powder bed, and most preferred at a distance from theouter edges of the polymeric powder bed of at most 10% of the totalwidth of the polymeric powder bed.

If desired, prior to feeding and compression-moulding the polymerpowder, the polymer powder may be mixed with a suitable liquid organiccompound having a boiling point higher than the melting point of saidpolymer. Compression moulding is preferably carried out by temporarilyretaining the polymer powder between the endless belts while conveying.This may for instance be done by providing pressing platens and/orrollers in connection with the endless belts. The UHMWPE polymer used inthis process is preferably drawable in the solid state.

Drawing, preferably uniaxial drawing, of the compression moulded polymermay be carried out by means known in the art. Such means compriseextrusion stretching and tensile stretching on suitable drawing units.To attain increased mechanical strength and stiffness, drawing may becarried out in multiple steps. In case of the preferred ultra highmolecular weight polyethylene films, drawing is typically carried outuniaxially in a number of drawing steps. The first drawing step may forinstance comprise drawing to a stretch factor of 3. Multiple drawing maytypically result in a stretch factor of 9 for drawing temperatures up to120° C., a stretch factor of 25 for drawing temperatures up to 140° C.,and a stretch factor of 50 for drawing temperatures up to and above 150°C. By multiple drawing at increasing temperatures, stretch factors ofabout 50 and more may be reached.

Since the polymeric tape of the invention is produced by providing clearboundaries for the polymeric powder bed, e.g. in the form of the easilycompressible strips, the tape is more uniform than known hitherto, inparticular in the transverse direction of the tape as produced.Polymeric tape of the invention may be obtained having further an arealweight varying less than 10% on average in the transverse direction ofthe tape, and preferably less than 5% on average in the transversedirection of the tape. Such more uniform tapes provide better, or atleast more consistent mechanical properties than the known tapes.

A preferred second process for the formation of films or tapes comprisesfeeding a polymer to an extruder, extruding a film or a tape at atemperature above the melting point thereof and drawing the extrudedfilm or tape. If desired, prior to feeding the polymer to the extruder,the polymer may be mixed with a suitable liquid organic compound, forinstance to form a gel, such as is preferably the case when using ultrahigh molecular weight polyethylene. Preferably the polyethylene filmsare prepared by such a gel process. A suitable gel spinning process isdescribed in for example GB-A-2042414, GB-A-2051667, EP 0205960 A and WO01/73173 A1, and in “Advanced Fibre Spinning Technology”, Ed. T.Nakajima, Woodhead Publ. Ltd (1994), ISBN 1 85573 182 7. In short, thegel spinning process comprises preparing a solution of a polyolefin ofhigh intrinsic viscosity, extruding the solution into a film at atemperature above the dissolving temperature, cooling down the filmbelow the gelling temperature, thereby at least partly gelling the film,and drawing the film before, during and/or after at least partialremoval of the solvent.

Drawing, preferably uniaxial drawing, of the produced films or tapes maybe carried out by means known in the art. Such means comprise extrusionstretching and tensile stretching on suitable drawing units. To attainincreased mechanical strength and stiffness, drawing may be carried outin multiple steps. In case of the preferred ultra high molecular weightpolyethylene films, drawing is typically carried out uniaxially in anumber of drawing steps. The first drawing step may for instancecomprise drawing to a stretch factor of 3. Multiple drawing maytypically result in a stretch factor of 9 for drawing temperatures up to120° C., a stretch factor of 25 for drawing temperatures up to 140° C.,and a stretch factor of 50 for drawing temperatures up to and above 150°C. By multiple drawing at increasing temperatures, stretch factors ofabout 50 and more may be reached. This results in high strength tapes,whereby for tapes of ultra high molecular weight polyethylene, strengthsof 1.5 GPa to 1.8 GPa and more may be obtained.

According to the invention, the resulting tapes, preferably theresulting drawn tapes, may be used as such to produce the material sheetby weaving, if their variation in width is less than 2% on average inthe longitudinal direction of the tape, and preferably less than 1% onaverage in the longitudinal direction of the tape. Alternatively, thetapes and in particular the drawn tapes as produced may be cut to theirdesired width, or split along the direction of drawing, to obtain thelimited width variation as required by the invention. Preferably thematerial sheet is woven from tape that is not slitted e.g. to form fiberlike structures as disclosed in U.S. Pat. No. 5,091,133.

The width of the tapes of the invention and in particular the width ofthe unidirectional tapes, is only limited by the width of the film fromwhich they are produced. The width of the tapes preferably is more than2 mm, preferably more than 5 mm, more preferably more than 10 mm, evenmore preferably more than 20 mm. Most preferably the width of the tapesis more than 40 mm. It was observed that wider tapes perform better whenwoven into material sheets and furthermore, material sheets comprisingwider tapes have further improved properties, in particularantiballistic properties, especially when the width of the tapes is morethan 40 mm. In principle there is no restriction to the maximum width ofthe tape. For practical reasons the preferred maximum width is at most400 mm, more preferably at most 300 mm, most preferably at most 200 mm.

The areal density of the tapes of the invention can be varied over alarge range, for instance between 5 and 200 g/m². Preferred arealdensity is between 8 and 120 g/m², more preferred between 10 and 80 g/m²and most preferred between 12 and 60 g/m², most preferred between 12 and30 g/m². The areal density of a tape can be determined by weighing aconveniently cut surface from the tape. It was observed that materialsheets made of such tapes have improved antiballistic performance.

The thickness of the tapes of the invention, in particular theunidirectional tapes, can in principle be selected within wide ranges.Preferably however, the thickness of the tapes used in weaving thematerial sheet of the invention does not exceed 120 μm, more preferablydoes not exceed 50 μm, and most preferably is comprised between 5 and 29μm. A further preferred material sheet according to the invention ischaracterized in that the thickness of the tapes used to manufacturethereof is greater than 10 μm and does not exceed 50 μm, preferably doesnot exceed 100 μm and more preferably does not exceed 120 μm. Bylimiting the thickness of the tapes in a material sheet to the claimedthicknesses, sufficient antiballistic properties are surprisinglyachieved even with tapes having rather limited strengths. The skilledperson knows how to determine the thickness of the tape, e.g. with amicrometer.

The strength of the tapes of the invention, in particular the tapes inthe material sheet, largely depends on the polymer from which they areproduced, and on their (uniaxial) drawing or draw ratio. The strength ofthe tapes is at least 0.75 GPa, preferably at least 0.9 GPa, morepreferably at least 1.2 GPa, even more preferably at least 1.5 GPa, evenmore preferably at least 1.8 GPa, and even more preferably at least 2.1GPa, and most preferably at least 3 GPa. The unidirectional tapes arepreferably sufficiently interconnected to each other, meaning that thematerial sheets according to the invention hardly delaminate undernormal use conditions such as e.g. at room temperature.

The material sheet according to the invention may comprise tapes woveninto e.g. fabrics of any structure. Suitable woven fabric structures mayinclude plain weave, twill weave, basket weave, satin weave, crowfootweave, and others. Particularly preferred is a material sheet, whereinthe woven fabric has a plain weave structure. Such a structure offers astable material sheet, which is easily processed further. Also, thisembodiment shows excellent antiballistic performance, especially in astand alone configuration. Another preferred embodiment of the materialsheet comprises a woven fabric having a twill weave structure. Such anembodiment is preferred in ballistic resistant articles, comprisingmaterial sheets of the invention and a further sheet of inorganicmaterial selected from the group consisting of ceramic, steel, aluminum,magnesium titanium, nickel, chromium and iron or their alloys, glass andgraphite, or combinations thereof. The present embodiment preferablycomprises a twill weave structure with an interlacing frequency rangingfrom 3-30:1, and more preferably ranging from 7-21:1. An interlacingfrequency of x:1 means that a warp (or weft yarn) crosses over x weft(or warp) yarns.

The material sheet of the invention may also include a binder which islocally applied to bond and stabilise the plurality of the tapes, inparticular unidirectional tapes used in manufacturing thereof, such thatthe structure of the material sheet is retained during handling andproducing of structures, e.g. antiballistic structures. Suitable bindersare described in e.g. EP 0191306 B1, EP 1170925 A1 , EP 0683374 B1 andEP 1144740 A1 . The binder may be applied in various forms and ways; forexample as a transverse bonding strip (transverse with respect to thee.g. unidirectional tapes). The application of the binder during theformation of the material sheet advantageously stabilises the tapes,thus enabling faster production cycles to be achieved.

In one embodiment, a binder is applied to fixedly abut adjacentunidirectional tapes along their longitudinal edges. As the role of thebinder is to temporarily retain and stabilise the plurality ofunidirectional tapes during handling and making of material sheets, e.g.antiballistic material sheets, localised application of the binder ispreferred. Local application of the binder is application that isrestricted to the immediate vicinity of the longitudinal edges and mayinclude intermittent localised application (spot application along thelongitudinal edges).

In still another preferred embodiment of the material sheet according tothe invention, the unidirectional tapes in the weft and/or warpdirection are mutually bonded, at least over an area adjacent to thelongitudinal edges of the woven fabric. In a particularly preferredembodiment, the unidirectional tapes of the drawn polymer in the warpand weft direction of the woven fabric are at least partly adhered toeach other, at least over an area adjacent to the longitudinal edges ofthe woven fabric by fusion bonding. In this embodiment, welding may beused for instance to intermittently fuse sections of the longitudinaledges of the material sheet together.

In embodiments with intermittent localised fusion of the unidirectionaltapes in the weft and/or warp direction, the proportion of thelongitudinal edges of the material sheet comprising intermittentlocalised fusion is preferably less than 50%, 30%, 20% 10%, 5% or 2%.When using a binder, the proportion of the longitudinal edges (or areasadjacent to the longitudinal edges) of the material sheet which israised due to the application of the binder is preferably less than 50%,30%, 20% 10%, 5% or 2%. Preferably, the binder comprises less than 20%,10%, 5%, 2% 1%, 0.5%, or 0.2% of the weight of the material sheet.

The material sheet according to the invention can be used in the form ofone woven structure, e.g. fabric, as produced. However, it is alsopossible to provide a multilayered material sheet by stacking aplurality of material sheets according to the invention (e.g. wovenfabrics). Such a multilayered material sheet preferably comprises atleast 2 woven fabrics, preferably at least 4 woven fabrics, morepreferably at least 6 woven fabrics, even more preferably at least 8woven fabrics, and most preferably at least 10 woven fabrics. Increasingthe number of woven fabrics in the multilayer material sheet of theinvention simplifies the manufacture of articles from these materialsheets, for instance antiballistic plates.

In one embodiment of the present invention, there is provided a processfor the preparation of a material sheet comprising:

-   (a) providing a plurality of drawn polymer tapes, preferably    unidirectional tapes, having a width that varies less than 2% on    average in the longitudinal direction of the tape;-   (b) weaving said plurality of drawn polymer tapes to form a woven    fabric;-   (c) compressing the thus formed woven fabric at least over an area    adjacent to the longitudinal edges of the woven fabric to    consolidate the area.

In another embodiment, the process is characterised in that, prior tostep (c) the unidirectional tapes of the drawn polymer in the warp andweft direction of the woven fabric are at least partly adhered to eachother, at least over an area adjacent to the longitudinal edges of thewoven fabric, an example of which is depicted in FIG. 2. In stillanother preferred process according to the invention, adhering theunidirectional tapes is performed by fusion bonding, and even morepreferably by ultrasonic welding.

The material sheet according to the invention is particularly useful inmanufacturing ballistic resistant articles, such as vests or armouredplates. Particularly good results are obtained when drawn tapes,preferably unidirectional tapes according to the invention are used inmanufacturing the material sheet. Ballistic applications compriseapplications with ballistic threat against projectiles of several kindsincluding against armor piercing, so-called AP bullets and hardparticles such as e.g. fragments and shrapnel. The material sheetaccording to the invention is most suitable for use in hard ballistics,such as e.g. panels, for use in vehicles for land/air or sea, or panelsfor inserts in bullet resistant vests. The invention therefore alsorelates to the enumerated ballistic resistant articles comprising thematerial sheet of the invention.

The ballistic resistant article according to the invention comprises atleast 1 woven fabric layer, preferably at least 5 woven fabric layers,more preferably at least 10 woven fabric layers, even more preferably atleast 15 woven fabric layers and most preferably at least 20 wovenfabric layers.

Preferably the ballistic resistant article according to the inventioncomprises a further sheet of inorganic material selected from the groupconsisting of ceramic; metal; glass; graphite, or combinations thereof.Particularly preferred is metal and in particular a metal having amelting point of at least 350° C., more preferably at least 500° C.,most preferably at least 600° C. Suitable metals include aluminum,magnesium, titanium, copper, nickel, chromium, beryllium, iron andcopper including their alloys as e.g. steel and stainless steel andalloys of aluminum with magnesium (so-called aluminum 5000 series), andalloys of aluminum with zinc and magnesium or with zinc, magnesium andcopper (so-called aluminum 7000 series). In said alloys the amount ofe.g. aluminum, magnesium, titanium and iron preferably is at least 50 wt%. Preferred metal sheets comprising aluminum, magnesium, titanium,nickel, chromium, beryllium, iron including their alloys. Morepreferably the metal sheet is based on aluminum, magnesium, titanium,nickel, chromium, iron and their alloys. This results in a lightantiballistic article with a good durability. Even more preferably theiron and its alloys in the metal sheet have a Brinell hardness of atleast 500. Most preferably the metal sheet is based on aluminum,magnesium, titanium, and their alloys. This results in the lightestantiballistic article with the highest durability. Durability in thisapplication means the lifetime of a composite under conditions ofexposure to heat, moisture, light and UV radiation. Although the furthersheet of material may be positioned anywhere in the stack of wovenfabric layers, the preferred ballistic resistant article ischaracterized in that the further sheet of material is positioned at theoutside of the stack of woven fabric layers, most preferably at least atthe strike face thereof.

The ballistic resistant article according to the invention preferablycomprises a further sheet of the above described inorganic materialhaving a thickness of at most 100 mm. Preferably the maximum thicknessof the further sheet of inorganic material is 75 mm, more preferably 50mm, and most preferably 25 mm. This results in the best balance betweenweight and antiballistic properties. Preferably, in the event that thefurther sheet of inorganic material is a metal sheet, the thickness ofthe metal sheet, is at least 0.25 mm, more preferably at least 0.5 mm,and most preferably at least 0.75 mm. This results in even betterantiballistic performance.

The further sheet of inorganic material may optionally be pre-treated inorder to improve adhesion with the multilayer material sheet. Suitablepre-treatment of the further sheet includes mechanical treatment e.g.roughening or cleaning the surface thereof by sanding or grinding,chemical etching with e.g. nitric acid and laminating with polyethylenefilm.

In another embodiment of the ballistic resistant article a bondinglayer, e.g. an adhesive, may be applied between the further sheet andthe multilayer material sheet. Such adhesive may comprise an epoxyresin, a polyester resin, a polyurethane resin or a vinylester resin. Inanother preferred embodiment, the bonding layer may further comprise awoven or non woven layer of inorganic fiber, for instance glass fiber orcarbon fiber. It is also possible to attach the further sheet to themultilayer material sheet by mechanical means, such as e.g. screws,bolts and snap fits. In the event that the ballistic resistant articleaccording to the invention is used in ballistic applications where athreat against AP bullets, fragments or improvised explosive devices maybe encountered the further sheet is preferably comprises a metal sheetcovered with a ceramic layer. In this way an antiballistic article isobtained with a layered structure as follows: ceramic layer/metalsheet/at least two unidirectional sheets with the direction of thefibers in the unidirectional sheet at an angle a to the direction of thefibers in an adjacent unidirectional sheet. Suitable ceramic materialsinclude e.g. alumina oxide, titanium oxide, silicium oxide, siliciumcarbide and boron carbide. The thickness of the ceramic layer depends onthe level of ballistic threat but generally varies between 2 mm and 30mm. This ballistic resistant article is preferably positioned such thatthe ceramic layer faces the ballistic threat.

In one embodiment of the present invention, there is provided a processfor the manufacture of a ballistic resistant article comprising:

-   (a) stacking at least 1 woven fabric layer of unidirectional tapes    of drawn polymer, wherein the width of a tape varies less than 2% on    average in the longitudinal direction of the tape; and a sheet of    material selected from the group consisting of ceramic, steel,    aluminum, titanium, glass and graphite, or combinations thereof; and-   (b) consolidating the stacked sheets under temperature and pressure.

In an alternative process a stack of at least 2 woven fabric layers ofunidirectional tapes of drawn polymer, wherein the width of a tapevaries less than 2% on average in the longitudinal direction of the tapeis manufactured in a separate process, such as has been described above.This pre-manufactured stack is then combined with the further sheet ofmaterial selected from the group consisting of ceramic, steel, aluminum,titanium, glass and graphite, or combinations thereof, in step (a) ofthe process.

Consolidation for all processes described above may suitably be done ina hydraulic press. Consolidation is intended to mean that the monolayersare relatively firmly attached to one another to form one unit. Thetemperature during consolidating generally is controlled through thetemperature of the press. A minimum temperature generally is chosen suchthat a reasonable speed of consolidation is obtained. In this respect80° C. is a suitable lower temperature limit, preferably this lowerlimit is at least 100° C., more preferably at least 120° C., mostpreferably at least 140° C. A maximum temperature is chosen below thetemperature at which the drawn polymer woven layers lose their highmechanical properties due to e.g. melting. Preferably the temperature isat least 10° C., preferably at least 15° C. and even more at least 20°C. below the melting temperature of the drawn polymer woven layer. Incase the drawn polymer woven layer does not exhibit a clear meltingtemperature, the temperature at which the drawn polymer woven layerstarts to lose its mechanical properties should be read instead ofmelting temperature. In the case of the preferred ultra high molecularweight polyethylene, a temperature below 145° C. generally will bechosen. The pressure during consolidating preferably is at least 7 MPa,more preferably at least 15 MPa, even more preferably at least 20 MPaand most preferably at least 35 MPa. In this way a stiff antiballisticarticle is obtained. The optimum time for consolidation generally rangesfrom 5 to 120 minutes, depending on conditions such as temperature,pressure and part thickness and can be verified through routineexperimentation. In the event that curved antiballistic articles are tobe produced it may be advantageous to first pre-shape the further sheetof material into the desired shape, followed by consolidating with themonolayers and/or multilayer material sheet.

Preferably, in order to attain a high ballistic resistance, coolingafter compression moulding at high temperature is carried out underpressure as well. Pressure is preferably maintained at least until thetemperature is sufficiently low to prevent relaxation. This temperaturecan be established by one skilled in the art. When a ballistic resistantarticle comprising monolayers of ultra high molecular weightpolyethylene is manufactured, typical compression temperatures rangefrom 90 to 150° C., preferably from 115 to 130° C. Typical compressionpressures range between 100 to 300 bar, preferably 100 to 180 bar, morepreferably 120 to 160 bar, whereas compression times are typicallybetween 40 to 180 minutes.

The multilayered material sheet and antiballistic article of the presentinvention are particularly advantageous over previously knownantiballistic materials as they provide at least the same level ofprotection as the known articles at a significantly lower weight, or animproved ballistic performance at equal weight compared with the knownarticle. Starting materials are inexpensive and the manufacturingprocess is relatively short and thus cost effective. Since differentpolymers may be used to produce the multilayered material sheet of theinvention properties may be optimized according to the particularapplication. Besides ballistic resistance, properties include forinstance heat stability, shelf-life, deformation resistance, bondingcapacity to other material sheets, formability, and so on.

It was also observed that the material sheet of the invention and theparticular constructions comprising said sheet as described above in theembodiments of the multilayered material sheet and of the ballisticresistant articles, are products particularly useful also inmanufacturing cargo panels, i.e. panels used in the construction ofcargo containers. Said products proved also particularly advantageous inmanufacturing construction walls; liners for e.g. cargo holds such asaircraft cargo holds; cargo pallet sheets and radomes. Furthermore, saidproducts and in particular the constructions of multilayered materialsheets and ballistic resistant articles proved extremely useful whenused to manufacture impact sensitive aircraft parts, e.g. wing edges,flaps or other prominent parts which are prone to suffer impacts frome.g. ice or birds. The invention therefore relates to the use of thematerial sheet of the invention in the above enumerated products andfurthermore to the above enumerated products comprising the materialsheet of the invention.

The invention moreover relates to the use of the tapes of the inventionin woven material sheets and also in a weaving process for manufacturingmaterial sheets.

The invention will now be further explained by the following FIGS. 1-4,without however being limited thereto.

FIG. 1 schematically represents an embodiment of a material sheetaccording to the invention.

FIG. 2 schematically represents another embodiment of a material sheetaccording to the invention.

FIG. 3 schematically represents still another embodiment of a materialsheet according to the invention.

FIG. 4 schematically represents a multilayer material sheet according tothe invention.

Referring to FIG. 1, a woven fabric of unidirectional tapes of drawnpolymer is shown. In the woven fabric, the width of the tapes of atleast 10 mm varies less than 2% on average in the longitudinal directionof the tapes. The woven fabric has been obtained by a weaving process asdescribed in WO2006/075961. After weaving the tapes according to a plainweave pattern (as shown in FIG. 1), the woven fabric is fed into a beltpress or calander press, known per se, for final consolidation of thematerial sheet. In the belt press or calander, the unidirectional tapesrunning in the warp and weft direction are bonded at a temperature closeto the melting point of the tapes. It should be noted that tapes of atleast 10 mm can be produced having a width varying less than 2% onaverage in the longitudinal direction of the tapes by drawing polymerfilms. In instances where this is not possible, a tape as produced issubsequently slitted along its longitudinal edges to obtain the limitedvariation in width, as required by the invention. Suitable slittingequipment is for instance a Bielloni Sage machine, model Taglierina,type RP/B1 505, equipped with chromium steel knives.

Referring to FIG. 2, another embodiment of a woven fabric ofunidirectional tapes of drawn polymer is shown. As in FIG. 1, the widthof the tapes of at least 10 mm varies less than 2% on average in thelongitudinal direction of the tapes. The woven fabric has been obtainedby a weaving process as described in WO2006/075961. After weaving thetapes according to a plain weave pattern, the woven fabric has beenpartly consolidated over an area adjacent to the longitudinal edges ofthe woven fabric only. The dots shown in FIG. 2 actually representlocations in which the unidirectional tapes have been fusion bonded, forinstance by welding.

Referring to FIG. 3, still another embodiment of a woven fabric ofunidirectional tapes of drawn polymer is shown. As in the previousfigures, the width of the tapes varies less than 2% on average in thelongitudinal direction of the tapes. The woven fabric has been obtainedby a weaving process as described in WO2006/075961, and consolidated ina belt press. The woven structure of this embodiment corresponds to atwill weave with an interlacing frequency of 3, i.e. a weft (warp) tapecrosses over 3 warp (weft) tapes.

Referring to FIG. 4, a graphical presentation of a multilayer materialsheet according to the invention is shown. The multilayer material sheetcomprises a woven fabric layer of FIG. 1 denoted as number 1 (in fulllines), with below it a second woven fabric layer denoted as number 2(in dotted lines). The second woven fabric layer is positioned such thatthe seam lines of the respective woven fabric layers are aligned in astaggered fashion.

Test methods as referred to in the present application, are as follows

-   -   Intrinsic Viscosity (IV) is determined according to method        PTC-179 (Hercules Inc. Rev. Apr. 29, 1982) at 135° C. in        decalin, the dissolution time being 16 hours, with DBPC as        anti-oxidant in an amount of 2 g/l solution, by extrapolating        the viscosity as measured at different concentrations to zero        concentration;    -   Tensile properties of yarn (measured at 25° C.): tensile        strength (or strength), tensile modulus (or modulus) and        elongation at break (or eab) are defined and determined on        multifilament yarns as specified in ASTM D885M, using a nominal        gauge length of the fiber of 500 mm, a crosshead speed of        50%/min. Tensile properties of tape (measured at 25° C.):        tensile strength (or strength), tensile modulus (or modulus) and        elongation at break (or eab) are defined and determined on tapes        of a width of 20 mm as specified in ASTM D882, using a nominal        gauge length of the tape of 440 mm, a crosshead speed of 50        mm/min.    -   Width variation of the tape, is determined by measuring the        largest width L and the smallest width S of a tape of a length        of 20 m (or alternatively 20 tapes of a length of 1 m). The        variation is L-S devided by S, expressed as percentage.    -   The melting point of a polymer is determined by DSC on a        power-compensation PerkinElmer DSC-7 instrument which is        calibrated with indium and tin with a heating rate of 10°        C./min. For calibration (two point temperature calibration) of        the DSC-7 instrument about 5 mg of indium and about 5 mg of tin        are used, both weighed in at least two decimal places. Indium is        used for both temperature and heat flow calibration; tin is used        for temperature calibration only.    -   The furnace block of the DSC-7 is cooled with water, with a        temperature of 4° C. in order to provide a constant block        temperature, for a stable baselines and good sample temperature        stability. The temperature of the furnace block should be stable        for at least one hour before the start of the first analysis.        For tape measurements, the tape is cut into small square pieces        of 5 mm maximum and a sample size of at least about 1 mg (+/−0.1        mg) is taken. Typically, for a tape with a thickness of 40        micron, one square piece of 5 mm is about 1 mg. For smaller        thicknesses more pieces are stacked. For thicker tapes the size        may be reduced, such that 1 mg sample mass is obtained at        minimum.    -   The represenative sample is put into an aluminum DSC sample pan        (50 μl), which is covered with an aluminum lid (round side up)        and then sealed. In the sample pan (or in the lid) a small hole        must be perforated to avoid pressure build-up (leading to pan        deformation and therefore a worsening of the thermal contact).        For powder samples, a minimum of 1 mg (+/−0.1 mg) of powder is        taken and charged into the sample pan.    -   The sample pan is placed in a calibrated DSC-7 instrument. In        the reference furnace an empty sample pan (also covered with a        pierced lid and sealed) is placed.    -   The following temperature program is run:        -   1. sample is kept for 5 min at 40° C. (stabilization period)        -   2. increase temperature from 40 up to 200° C. with 10°            C./min. (first heating curve)        -   3. sample is kept for 5 min at 200° C.        -   4. temperature is decreased from 200 down to 40° C. (cooling            curve)        -   5. sample is kept for 5 min at 40° C.        -   6. optionally increase temperature from 40 up to 200° C.            with 10° C./min to obtain a second heating curve.    -   The same temperature program is run with an empty pan in the        sample side of the DSC furnace (empty pan measurement).    -   Analysis of the first heating curve is used as known in the art        to determine the melting temperature of the analyzed sample. The        empty pan measurement is subtracted from the sample curve to        correct for baseline curvature. Correction of the slope of the        sample curve is performed by aligning the baseline at the flat        part before and after the peaks (at 60 and 190° C. for UHMWPE).        The peak height is the distance from the baseline to the top of        the peak.        The invention is now further explained by means of the following        example, without being limited hereto.

EXAMPLE I Ia—Production of Tape

A ultrahigh molecular weight polyethylene powder as described in WO93/15118 having a bulk density of 275 kg/m³ and an active catalystresidue of 47 ppm was fed into a powder bed of a width of 30 cm. Thisbed was heated to a temperature of 135° C. and pressed at a pressure of35 bar during 1 minute. The obtained tape precursor was calandered at atemperature of 140° C., i.e. below the melting point of the powder, andsubsequently drawn to a total draw ratio of 150, to form a tape.

The tape as produced had a tenacity of 1.7 GPa, measured on a small (20mm) slit tape. The tape had a width of about 60 mm, and was slit to awidth of 50.5±0.5 mm, using a Bielloni Sage machine, model Taglierina,type RP/B1 505, equipped with chromium steel knives. Thickness of thetape was 37 μm.

Ib—Production of Woven Fabric Material

The tapes were converted into a woven fabric with a plain weavestructure, as shown in FIG. 1. The tape woven structure had a width of130 cm and was stabilized by fusion at the edges of the product, asshown in FIG. 2. Without stabilizing the “fabric” it tends to fall apartwhen cutting it into ballistic panel sized sheets. The woven fabric thusproduced was then fed to a lamination line (manufactured by Meyer®),which is a belt press having different temperature and pressure zones.The heating zone was set to a temperature of 146° C., followed bycoaling. Pressure: 18 N/cm2. Total residence time was 2 minutes.

Ic—Production of Armor Panels from the Tape Panels were made of size50×50 cm. A first layer of woven fabric was placed on a surface. Asecond layer of woven fabric was placed on top of the first layer, andin such fashion that the seam lines of the two layers were positioned ina staggered manner. The procedure was repeated until an areal density(AD) of 8 kg/m² was reached. The stack was then supplemented withcommercially available 8 mm AL2O3 tiles (50 mm×50 mm tiles) having apurity of at least 98%. The stack was then moved into a press andpressed at a temperature of 145° C. and a pressure of 165 bar for 40minutes. Cooling was performed under pressure until a temperature of 80°C. was reached. Total cycle time was about 70 minutes.

Id—Performance Testing of Armored Panels

The armoured plates were subjected to shooting tests performed with7.62×51 mm AP-M2 (St. Louis Ordnance Plant, Missouri, USA) bullets. Thetests were performed with the aim of determining the V50 value. V50 isthe speed at which 50% of the projectiles will penetrate the armouredplate. The testing procedure was as follows.

The first projectile was fired at the anticipated V50 speed. The actualspeed was measured shortly before impact. If the projectile was stopped,a next projectile was fired at an intended speed of about 20 m/s higher.If it perforated, the next projectile was fires at an intended speed ofabout 20 m/s lower. The actual speed of impact was always measured. V50was the average of the two highest stops and the two lowestperforations.

Comparative Experiment A

-   -   Production of Armor panels    -   The same procedure was used for the manufacture of the panels of        example I whereby instead of the woven fabric Dyneema® HB26 (DSM        Dyneema, Netherlands) was used. This is a commercially available        material based on crossplied unidirectional polyethylene fibers    -   Performance testing of amored panels    -   Was done in the same way as for Example I.

Results:

Ex. Strike face Backing V50 m/s I 8 mm AL2O3 8 kg/m2 888 plain wovenfabric A 8 mm AL2O3 8 kg/m2 862 Dyneema ® HB26

The results confirm that a ballistic article comprising a material sheetaccording to the invention comprising a woven fabric of drawnpolyethylene produces unexpectedly improved anti-ballistic performance.This is the more surprising since it is common knowledge that hithertoknown woven fabrics show lower ballistic protection that thecommercially available products based on crossplied unidirectionallyaligned polyethylene fibers.

In particular, the ballistic article of the present invention produced asignificantly higher V50 value than is known from the prior art.

1. A process for the preparation of a polymeric tape having a width thatvaries less than 2% on average in the longitudinal direction of thetape, the process comprising forming a polymeric powder bed,compression-moulding the polymeric powder bed at a temperature below themelting point of the polymeric powder, and wherein the powder bed ismoulded by compression together with at least one compressible borderelement positioned onto the powder bed to partition said powder bed intoat least two parts, said compressible bordering element being made of acompressible material.
 2. A process according to claim 1, comprisingdrawing the compression-moulded polymer.
 3. A process according to claim2, wherein drawing is carried out in multiple steps.
 4. A processaccording to claim 3, comprising multiple drawing at increasingtemperatures.
 5. A process according to claim 1, wherein the polymerfrom which the polymeric tape is made is selected from the groupconsisting of polyolefins, polyesters, polyvinyl alcohols,polyacrylonitriles, and polyamides.
 6. A process according to claim 1,wherein the polymeric tape comprises ultra high molecular weightpolyethylene.
 7. A process according to claim 6, wherein the ultra highmolecular weight polyethylene polymer used in this process is drawablein the solid state.
 8. A process according to claim 1, wherein the atleast one compressible border element includes at least one compressiblestrip made of a material having a melting temperature as determined byDSC of at least 10° C. higher than the melting temperature of thepolymeric powder.
 9. A process according to claim 8, wherein thecompressible strip is formed of a thermoplastic polymer.
 10. A processaccording to claim 8, wherein the compressible strip is formed frompolypropylene, rubber polymer or silicon rubber.
 11. A process accordingto claim 8, wherein the compressible strip has a rectangular,triangular, circular, or polygonal cross-section.
 12. A processaccording to claim 8, wherein the compressible strip comprises a hose ora tube.
 13. A process according to claim 12, wherein the compressiblestrip has a ratio of outer diameter: inner diameter of 3:2, 3:1.5 or3:1.
 14. A process according to claim 8, wherein the compressible striphas an undeformed height which is equal or higher than an undeformedaverage thickness of the polymeric powder bed.